© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 7137 wileyonlinelibrary.com COMMUNICATION Terahertz Magnetic Mirror Realized with Dielectric Resonator Antennas Daniel Headland, Shruti Nirantar, Withawat Withayachumnankul, Philipp Gutruf, Derek Abbott, Madhu Bhaskaran, Christophe Fumeaux,* and Sharath Sriram* D. Headland, Dr. W. Withayachumnankul, Prof. D. Abbott, Prof. C. Fumeaux School of Electrical & Electronic Engineering The University of Adelaide Adelaide, South Australia 5005, Australia E-mail: cfumeaux@eleceng.adelaide.edu.au S. Nirantar, Dr. W. Withayachumnankul, P. Gutruf, Dr. M. Bhaskaran, Dr. S. Sriram Functional Materials and Microsystems Research Group School of Electrical and Computer Engineering RMIT University Melbourne, Victoria 3000, Australia E-mail: sharath.sriram@gmail.com S. Nirantar, P. Gutruf, Dr. M. Bhaskaran, Dr. S. Sriram Micro Nano Research Facility RMIT University Melbourne, Victoria 3000, Australia Dr. W. Withayachumnankul Interdisciplinary Graduate School of Science and Engineering Tokyo Institute of Technology Ookayama 152-8552, Meguro-ku, Tokyo, Japan DOI: 10.1002/adma.201503069 PMC is at a local maximum of absolute electric field strength. An object placed close (<< λ/4) to the surface of a PMC will therefore have a stronger interaction with electric fields than an object placed close to the surface of a PEC. This unique PMC property can be exploited in a broad range of applications from antennas, [1–4] sensing platforms, [5] to optical components. Magnetic conductors do not naturally occur, but their response can be approximated using structured surfaces, termed artificial magnetic conductors (AMCs), or magnetic mirrors. [6–8] Such a response is typically achieved with arrays of resonant elements, such as metallic resonators supported by a dielectric layer and a ground plane. [8–11] Each resonator, together with the ground plane, forms a magnetic dipole that exhibits magnetic phase reversal near resonance. Thus, AMCs mimic the response of a PMC within a certain operational bandwidth, with near-unity reflectivity and a near-zero phase change of the electric field upon reflection. Metallic resona- tors are well covered in the microwave range, as they form the basis for conventional reflectarrays. [12–18] They remain an active field of research in the terahertz [19–23] and optical frequency ranges, [24–26] but are increasingly lossy at such frequencies, given the non-negligible loss of the Drude metals. At terahertz frequencies, there is an additional challenge of identifying low- loss dielectrics suitable to support the metallic resonators. [27] To bypass ohmic heat dissipation, resonant dielectric structures are preferable for functional and efficient AMCs at terahertz and optical frequencies. [28–30] Inside a dielectric structure of moderate to high rela- tive permittivity, electromagnetic radiation is confined into standing waves, or resonant modes. The resonance frequency is dependent on the geometry and material properties of the dielectric resonator (DR). If the DR is unshielded, the modes can couple with free space radiation, [31] and if the DR is utilized as a radiator in this way, it can be termed a dielectric resonator antenna (DRA). [32,33] DRAs are theoretically scalable across a broad range of frequencies, [34] and have been demonstrated in the visible range. [35] Key to the operation of the DRA is the quality factors: Q diss is associated with energy lost to dissipa- tion and Q rad is associated with energy radiated to free space. In the majority of cases, it is beneficial for a DRA to have high Q diss in order to ensure efficient operation, and hence low-loss dielectrics are sought to construct DRAs. However, the choice of Q rad depends on the desired application. High Q rad is gen- erally beneficial for sensing applications, in order to maximize resonator sensitivity. Such high Q rad DRAs, using a dielectric of high relative permittivity, have previously been employed in the terahertz range as resonators for a metamaterial application. [36] However, a drawback of high Q rad is narrowband operation. An array of highly efficient terahertz passive dielectric resonator antennas (DRAs) is investigated. The realization of this device requires relatively thick, single-crystal silicon to be incorporated on a metal film. To this end, an unconventional microfabrica- tion procedure is developed, which makes use of a combination of SU-8-assisted bonding, photolithography, and deep reactive ion etching. The fabricated DRAs exhibit a magnetic dipole mode of resonance, and hence the device behaves as a mag- netic mirror, with a 30% useful bandwidth. The efficiency of the DRA array is determined by numerical simulation to be 97% on resonance at 0.8 THz. This experimental demonstra- tion of DRAs at terahertz frequencies opens opportunities for highly efficient terahertz components with impact in the areas of imaging, sensing, and communications. Essentially, an electromagnetic wave reflected from a per- fect electrical conductor (PEC) experiences a 180° phase shift in its electric field component. Owing to this phase shift, the standing wave that is produced from the interference between the incident and reflected waves results in zero absolute elec- tric field strength at the PEC surface. A consequence of this process is that strong field–matter interaction can only occur at a minimum distance away from a PEC surface. As a coun- terpart of a PEC, a perfect magnetic conductor (PMC) imposes a zero phase change on the electric component of the reflected wave, and the magnetic component undergoes phase reversal. In this case, the incident and reflected electric fields close to the surface of a PMC are in phase. Hence, the surface of the Adv. Mater. 2015, 27, 7137–7144 www.advmat.de www.MaterialsViews.com